Modified asphalt binder material using crosslinked crumb rubber and methods of manufacturing the modified asphalt binder

ABSTRACT

Bituminous asphalt binder materials which are modified by the addition of crumb rubber or ground tire rubber and a cross-linking agent are described. In addition, methods are provided for producing a modified asphalt binder containing crumb rubber or ground tire rubber and a cross-linking agent. The modified asphalt binders comprise neat asphalt, crumb rubber, one or more acids and a cross-linking agent. Optionally, the modified asphalt binder may include one or more polymer additives. The crumb rubber may be obtained from recycled truck and/or automobile tires. The addition of crumb rubber in asphalt binders can improve the consistency and properties of the asphalt binders at high and low temperatures. In particular, the modified asphalt binders of the present invention exhibit improved elastic behavior, resulting in improved performance of roads or other surfaces paved using the modified asphalt binder. Road resistance to permanent deformation, fatigue cracking and thermal cracking is improved by use of the modified asphalt binder.

CROSS-REFERENCE TO PRIORITY APPLICATION

This application claims priority pursuant to 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/677,402 filed on May 3, 2005entitled “Modified Asphalt Binder Material Using Crosslinked CrumbRubber and Methods of Manufacturing the Modified Asphalt Binder”, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed, in one aspect, to bituminous asphaltbinder materials which are modified by the addition of crumb rubber orground tire rubber and a cross-linking agent. In a second aspect, thepresent invention is directed to methods of producing a modified asphaltbinder containing crumb rubber or ground tire rubber and a cross-linkingagent. The modified asphalt binders of the present invention compriseneat asphalt, crumb rubber, one or more acids and a cross-linking agent.Optionally, the modified asphalt binder may include one or more polymeradditives. The crumb rubber may be obtained from recycled truck and/orautomobile tires.

The addition of crumb rubber in asphalt binders can improve theconsistency and properties of the asphalt binders at high and lowtemperatures. In particular, the modified asphalt binders of the presentinvention exhibit improved elastic behavior, resulting in improvedperformance of roads or other surfaces paved using the modified asphaltbinder. Road resistance to permanent deformation, fatigue cracking andthermal cracking is improved by use of the modified asphalt binder. Theaddition of the cross-linking agent may also improve the stability ofthe modified asphalt binder for storage.

BACKGROUND

As used herein and in the claims, the phrase “asphalt binder” refers toa bituminous material, sometimes referred to as bitumen, used as abinder in asphalts used to pave roads or other surfaces, or used inconstruction materials such as roofing materials, coatings, and watersealants. Examples of bitumen that may be used in the compositions andmethods of the present invention include natural bitumens, pyrobitumensand artificial bitumens. Bitumens that are particularly preferred arethose used for roadways, such as asphalt or maltha. Asphalt pavingmaterial is made by mixing the asphalt binder with aggregate.

As used herein and in the claims, the phrase “crumb rubber” refers torubber particles which have a particle size of less than about 5 mm, andpreferably have a particle size of less than about 2 mm. Crumb rubbermay be obtained from grinding of used truck tires or automobile tires,or from any other appropriate source of ground rubber.

The use of crumb rubber and polyphosphoric acid in asphalt binders wasdescribed previously in publication number WO 04/081098, titled“Bituminous Binder and Method for the Production Thereof.” As describedin that published patent application, by combining between 0.5% byweight to 5% by weight polyphosphoric acid, and between 0.5% by weightto 25% by weight crumb rubber (or ground tire rubber) with thebituminous asphalt binder, the properties of the asphalt binder may beadvantageously modified without increasing the rotational viscosity suchthat the mixing process requires high temperature conditions.

Asphalt binders are frequently used in applications where there can be awide variation in environmental conditions, particularly when used inpavements. Accordingly, the properties of the asphalt binder in high andlow temperature conditions is a concern. At low temperatures, somebinder materials can become brittle, leading to long transverse fissuresdue to thermal stress. At higher temperatures, the asphalt binderbecomes more fluid (i.e. the viscosity is lower), which can lead torutting of a pavement due to the passage of vehicles over the surface.Resistance to fatigue and impact, and the adherence of the asphaltbinder to aggregate in paving applications, are properties of aparticular binder that also must be considered in particularapplications.

Some asphalt binders may require a relatively high elastic behavior, forexample where the corresponding asphalt paving mixture is used in areasof high traffic rates and high loads. Crumb rubber (or ground tirerubber), used alone or used in combination with polyphosphoric acid,does not sufficiently improve the elastic behavior of the asphalt pavingmixture for high traffic and high load uses. When a high elasticity isrequired, large amounts of crumb rubber must be added to the asphaltbinder. This can cause an undesirable increase in rotational viscosity,as well as problems related to storage of the binder material.

Accordingly, among the objects of the present invention is to provide anasphalt binder material with a relatively high elasticity, an acceptablerotational viscosity, and that can be stored for adequate periods oftime. Another object of the present invention is to provide methods ofmaking an asphalt binder having these properties.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a modified asphaltbinder material comprising asphalt, crumb rubber, one or more acids, across-linking agent, and, optionally, one or more polymers. In oneembodiment of the invention, neat asphalt is modified by adding 0.5% to30% by weight of crumb rubber, 0.05% to 5% by weight of one or moreacids, and 0.01% to 5% by weight of a cross-linking agent. The modifiedasphalt binder may also include between 0.5% by weight and 30% by weightof one or more polymer additives. The asphalt binders of the presentinvention typically have between about 10% to about 90% elastic recoveryunder a standard elastic recovery test, such as the test protocols setforth in AASHTO T51, ASTM D6084-04, NLT329 or other standard tests.

In another aspect, the present invention is directed to methods ofproducing a modified asphalt binder material comprising asphalt, crumbrubber, one or more acids, one or more cross-linking agents, and,optionally, one or more polymers.

In yet another aspect, the modified asphalt binders of the presentinvention may be mixed with water and an emulsifier to form a emulsion.The emulsified asphalt binder may be mixed with an aggregate material,spread to form a layer of the desired thickness, and the emulsion willbe broken to form an asphalt pavement. Alternatively, the emulsifiedasphalt binder may be spread upon a surface, an aggregate material maybe spread over the emulsified binder, and the emulsion may be broken.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to modified asphalt binders andmethods of making modified asphalt binders. The modified asphalt binderscomprise neat asphalt, crumb rubber, one or more acids, and one or morecross-linking agents. Optionally, the compositions may further includeone or more polymers. It will be understood that “crumb rubber” as usedherein includes crumb rubber, such as ground tire rubber or any otherrubber provided in particle form suitable for mixture with an asphaltbinder. Typically, a substantial portion of the crumb rubber will have aparticle size less than about 5 mm, preferably less than about 2 mm, andmore preferably less than 1 mm. The invention is not limited in thisregard, and the crumb rubber may have any particle size distributionthat results in an asphalt binder with the desired properties.

The modified asphalt binders of the present invention comprise betweenabout 60% by weight to about 98.9% by weight neat asphalt, between about0.1% by weight to about 30% by weight crumb rubber, between about 0.05%by weight to about 5% by weight of one or more acids, and between about0.01% by weight to about 5% by weight of a cross-linking agent.Optionally, the modified asphalt binder may further comprise betweenabout 0.5% by weight to about 30% by weight of one or more syntheticpolymers.

In one embodiment, the modified asphalt binder is comprised of betweenabout 82% by weight and 99% by weight neat asphalt, between about 0.5%by weight and 10% by weight crumb rubber, between about 0.5% by weightto about 3% by weight of one or more acids, and between about 0.01% byweight and 5% by weight of one or more cross-linking agents.

In another embodiment, the modified asphalt binder is comprised ofbetween about 52% by weight and 98.5% by weight neat asphalt, betweenabout 0.5% by weight and 10% by weight crumb rubber, between about 0.5%by weight to about 3% by weight of one or more acids, between about0.01% by weight and 5% by weight of one or more cross-linking agents,and between about 0.5% by weight to about 30% by weight of one or moresynthetic polymers.

Preferred acids for use in the modified asphalt binder of the presentinvention include phosphoric acid, polyphosphoric acid (more than 100%expressed as orthophosphoric content)(“PPA”), sulfuric acid at more than90% wt, boric acid, and carboxylic acids such as, for example, adipicacid, citric acid, oxalic acid, tartaric acid, maleic acid, valericacid, succinic acid, fumaric acid, glutamic acid, phtalic acid, aceticacid, and combinations of the above acids. The invention is not limitedin this regard, and any appropriate acid known to those skilled in theart may be used in the modified asphalt binder.

The acid may be added to the asphalt binder in either a solid form or ina liquid solution. Where a solid form of the acid is used, the acid canbe either a pure acid, such as boric acid or polyphosphoric acid, or theacidic component may be combined with an inert component for ease ofhandling, such as for example a SiO₂-PPA additive.

Preferred cross-linking agents include sulfur based compounds such as,for example, benzothiazoles, diphenylguanidine, dithiocarbamate, andelementary sulfur and/or a mixtures thereof. The butaphalt crosslinkeris also suitable, as are the croslinkers cited in the following UnitedStates patents and published applications: U.S. Pat. No. 6,451,886;application Ser. No. 2003144387 and U.S. Pat. No. 5,256,710. Theinvention is not limited in this regard and any appropriate rubbercross-linking agent known to those skilled in the art may be utilized inthe present invention.

In those embodiments of the present invention in which a syntheticpolymer is used, preferred synthetic polymers include styrene butadiene,styrene butadiene styrene (“SBS”) three block, ethylene vinyl acetate,ethylene propylene copolymers, polyvinylchorlide (PVC), nylon,polysterene, polybutadiene, acrylate resins, flurorocarbone resins,phenolic resins, alkyd resins, polyesters, polyethylene (linear orcrosslinked), epoxy terpolymer, polypropylene (attactic or isotactic),and combinations of the above polymers. The invention is not limited inthis regard, and any appropriate synthetic polymer known to thoseskilled in the art may be used in the modified asphalt binder.

In a second aspect, the present invention is directed to methods ofproducing the modified asphalt binder. For those embodiments of thepresent invention which do not include a synthetic polymer additive, thepreferred methods for manufacturing the modified asphalt binder comprisethe steps of (1) heating the asphalt to a temperature of between about120° C. and about 200° C., (2) adding a first modifying ingredient, (3)mixing the asphalt and the first modifying ingredient with a high shearmixer, such as, for example, a rotor-stator type mixer (i.e. a SILVERSONtype mixer) for a period of between about 5 minutes and about 10 hours,(4) adding a second modifying ingredient to the modified asphalt binder,(5) mixing the second modifying ingredient and the modified asphaltbinder in a high shear mixer for a period of between about 5 minutes andabout 10 hours, (6) adding a third modifying ingredient to the modifiedbinder material, and (7) agitating the third modifying ingredient andthe modified asphalt binder in a low shear mixer (such as, for example,a propeller type mixer driven by a motor at about 250 rpm, similar to anIKA type lab mixer) for a period of between about 5 minutes and about 48hours.

In these embodiments of the methods of the present invention, the firstmodifying ingredient may be either crumb rubber or one or more acids.Where the first modifying ingredient is crumb rubber, the secondmodifying ingredient is the cross-linking agent, and the third modifyingingredient is one or more acids. Alternatively, where the firstmodifying ingredient is one or more acids, the second modifyingingredient is crumb rubber and the third modifying ingredient is across-linking agent. Preferably, crumb rubber is added to the asphalt toachieve a crumb rubber level of between about 0.1% by weight and about30% by weight in the final modified asphalt material, one or more acidsare added to achieve a total acid concentration of between about 0.05%by weight and about 5% by weight in the modified asphalt material, andthe cross-linking agent is added to achieve a level of between about0.01% by weight to about 5% by weight of the cross-linking agent.

In other embodiments of the methods of the present invention, one ormore synthetic polymers are added to the modified asphalt composition.In these embodiments of the present invention, the preferred methods ofmodifying the asphalt binder generally include the steps of (1) heatingthe asphalt to a temperature of between about 120° C. and about 200° C,(2) adding a first modifying ingredient, (3) mixing the asphalt and thefirst modifying ingredient with a high shear mixer, such as, forexample, a rotor-stator type mixer (i.e. a SILVERSON type mixer) for aperiod of between about 5 minutes and about 10 hours, (4) adding asecond modifying ingredient to the modified asphalt binder, (5) mixingthe second modifying ingredient and the modified asphalt binder in ahigh shear mixer for a period of between about 5 minutes and about 10hours, (6) adding a third modifying ingredient to the modified bindermaterial, (7) optionally, mixing the third modifying ingredient and themodified asphalt binder for a period of between about 5 minutes andabout 10 hours, (8) adding a fourth modifying agent to the modifiedbinder material, and (9) agitating the fourth modifying ingredient andthe modified asphalt binder in a low shear mixer (such as, for example,a propeller type mixer driven by a motor at about 250 rpm, similar to anIKA type lab mixer) for a period of between about 5 minutes and about 48hours.

The modifying ingredients used in these embodiments of the methods arecrumb rubber, one or more acids, one or more synthetic polymers, and across-linking agent as described above. The crumb rubber, acids andsynthetic polymers may be added to the asphalt in any order. Thecross-linking agent is added to the asphalt after the crumb rubber hasbeen added, but the cross-linking agent may be added before or aftereither the acids or the polymers.

In these embodiments of the methods of the present invention, crumbrubber is added to the asphalt to achieve a crumb rubber level ofbetween about 0.1% by weight and about 30% by weight in the finalmodified asphalt material, one or more acids are added to achieve atotal acid concentration of between about 0.05% by weight and about 5%by weight in the modified asphalt material, one or more syntheticpolymers are added to achieve a total polymer concentration of betweenabout 0.5% by weight and about 30% by weight, and the cross-linkingagent is added to achieve a level of between about 0.01% by weight toabout 5% by weight of the cross-linking agent.

It will be understood by those skilled in the art that low shear mixersmay be used in place of high shear mixers in the methods described abovedepending upon the temperatures and the mixing times used, and oneskilled in the art can readily determine the appropriate mixing timesbased upon the temperature and the additive materials used.

The preferred synthetic polymers and the preferred acids used in themethods of the present invention are described above.

Several exemplary embodiments of the methods of the present inventionare described below:

Crumb Rubber—Acid—Cross-Linking Agent System

EXAMPLE 1

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

EXAMPLE 2

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

Crumb Rubber—Polymer—Acid—Cross-Linking Agent System

EXAMPLE 3

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.5% by weight to about 5% by weight of        one or more synthetic polymer    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

EXAMPLE 4

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.5% by weight to about 5% by weight of        one or more synthetic polymer    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with a high shear mixer for between about 5 minutes to 10        hours    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 h

EXAMPLE 5

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 5% by weight of        one or more synthetic polymers    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

EXAMPLE 6

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 5% by weight of        one or more synthetic polymers    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

EXAMPLE 7

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 5% by weight of        one or more synthetic polymers    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

EXAMPLE 8

-   -   Neat asphalt is heated to a temperature of between about 120° C.        to about 200° C.    -   Add from between about 0.5% by weight to about 3% by weight of        one or more acids    -   Mix with high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.1% by weight to about 10% by weight of        crumb rubber    -   Mix with a high shear mixer for between about 5 minutes to about        10 hours    -   Add from between about 0.5% by weight to about 5% by weight of        one or more synthetic polymers    -   Mix with a high shear mixer for between about 5 minutes and        about 10 hours    -   Add from between about 0.01% by weight to about 5% by weight of        a cross-linking agent    -   Agitate the modified asphalt obtained with low shear mixer from        5 minutes to 48 hours

Tests were conducted to measure the properties of modified asphaltbinders using cross-linking agents according to the present invention.In one set of tests, a modified asphalt binder was prepared using onlycrumb rubber and PPA, while a second modified asphalt binder wasprepared using crumb rubber, 0.1% sulfur as a cross-linking agent, andPPA. In both cases, crumb rubber was first added to the asphalt binderand stirred with a high shear mixer for about two hours. For the firstmodified binder, PPA was added and mixed using a high shear mixer forabout 30 minutes. For the second modified binder, sulfur was added andmixed using a high shear mixer for about 15 minutes, followed byaddition of PPA and further mixing with a high shear mixer for about 30minutes. The measured properties of the resulting modified asphaltbinders are summarized below in Table 1.

TABLE 1 % CR 5 5 % PPA 0.5 0.5 Cross-linker None 0.1% S Asphalt PG 64-22PG 64-22 Temperature 320° C. 160° C. Visc, cP, 135° C. 890 1020 ER, %,25° C. 35 45 Top end tru-grade 72 72.9 BBR, Stiffness, MPa 180 205 BBR,m-value 0.323 0.321

A series of tests were conducted in which a modified asphalt binder wasproduced by addition of SBS, polyphosphoric acid and crumb rubber to anasphalt binder. In one of the modified asphalt binders, a cross-linkingagent was added to the asphalt binder following the addition of thecrumb rubber. The mixing times using a high shear mixer were as follows(mixing times following the addition of each component was the sameregardless of the order of addition): following addition of crumbrubber, about 2 hours; following addition of SBS, about 6 hours;following addition of sulfur, about 30 minutes; following addition ofPPA, about 30 minutes. The properties of the modified asphalt bindersobtained in the tests are summarized in Table 2.

TABLE 2 Order of addition CR-Crosslinker- SBS-PPA-CR SBS-CR-PPA SBS-PPA% CR 5 5 5 % PPA 0.5 0.5 0.5 % SBS 1.0 1.0 1.0 Crosslinker, % None None1% SULFUR Temperature 160° C. 160° C. 160° C. Visc, cP, 135° C. 14001320 1320 ER, %, 25° C. 50 50 57.5 Top end tru-grade, 74.4 75.6 74.8 °C. BBR, Stiffness, 143 156 163 MPa BBR, m-value 0.329 0.326 0.321

As can be seen in the above tables, in each case, the modified asphaltbinder containing crumb rubber and a cross-linking agent demonstratedimproved elasticity compared to formulations that did not include across-linking agent.

The modified asphalt composition may be used in an emulsion type processto apply the asphalt binder material. In one embodiment, the emulsionprocess comprises the following steps:

-   1. the modified asphalt composition is prepared as described above;-   2. an emulsion of the modified asphalt composition obtained in step    1 is prepared by mixing water, the modified asphalt composition and    an emulsifier at ambient temperature;-   3. the emulsion obtained in step 2 is spread in order to obtain a    uniform layer of the emulsified asphalt binder; and-   4. the emulsion is broken.

Prior to breaking the emulsion, an aggregate material may be spread onthe emulsified asphalt binder. Alternatively, the process describedabove may include an additional step in which aggregate is added, withstirring and at ambient temperature, to the emulsion obtained in step 2of the process to form an asphalt pavement material. The asphaltpavement material is spread to the desired thickness and the emulsion isbroken. The emulsifier may be any appropriate emulsifier known to thoseskilled in the art. Also, the emulsion may be broken using conventionalmethods for breaking asphalt emulsions.

It will be recognized by those skilled in the art that the compositionsor methods described above may be altered in numerous ways withoutdeparting from the scope of the present invention. For example, one ormore of the mixing steps described above may be omitted, two or more ofthe modifying ingredients may be added to the asphalt together or at thesame time, or additional modifying agents may be added to thecomposition to further modify the properties of the composition.Accordingly, the preferred embodiments described herein are intended tobe illustrative rather than limiting in nature.

1. A method for making a modified asphalt binder composition, comprising the steps of: a. providing neat asphalt in an appropriate vessel; b. heating the neat asphalt to a temperature of between about 120° C. and about 200° C.; c. adding to the neat asphalt a first modifying ingredient selected from the group consisting of crumb rubber, at least one acid, and one or more synthetic polymers; d. mixing the asphalt and the first modifying ingredient with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours; e. adding a second modifying ingredient selected from the group consisting of crumb rubber, at least one acid, one or more synthetic polymers, and a cross-linking agent, wherein the second modifying ingredient is a different type of modifying ingredient than the first modifying ingredient, and the second modifying ingredient may be a cross-linking agent only if the first modifying ingredient is crumb rubber; f. mixing the second modifying ingredient and the asphalt with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours g. adding a third modifying ingredient selected from the group consisting of crumb rubber, at least one acid one or more synthetic polymers and a cross-linking agent, wherein the third modifying ingredient is a different type of modifying ingredient than the first modifying ingredient and the second modifying ingredient, and the third modifying ingredient may be a cross-linking agent only if the first or the second modifying ingredient is crumb rubber; h. agitating the third modifying ingredient and the asphalt with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 48 hours; i. adding a fourth modifying ingredient selected from the group consisting of at least one acid one or more synthetic polymers and a cross-linking agent, wherein the third modifying ingredient is a different type of modifying ingredient than the first modifying ingredient, the second modifying ingredient, and the third modifying ingredient; and j. agitating the fourth modifying ingredient and the asphalt with a low shear mixer for a period of between about 5 minutes and about 48 hours.
 2. The method of claim 1, wherein the first modifying ingredient is between about 0.5% by weight and about 30% by weight of at least one synthetic polymer, the second modifying ingredient is between about 0.1% by weight and about 30% by weight of crumb rubber, the third modifying ingredient is between about 0.5% by weight and about 5% by weight of at least one acid, and the fourth modifying ingredient is between about 0.01% and about 5% of a cross-linking agent.
 3. The method of claim 1, wherein the first modifying ingredient is between about 0.5% by weight and about 30% by weight of at least one synthetic polymer, the second modifying ingredient is between about 0.05% by weight and about 5% by weight of at least one acid, the third modifying ingredient is between about 0.1% by weight and about 30% by weight of crumb rubber, and the fourth modifying ingredient is between about 0.01% and about 5% of a cross-linking agent.
 4. The method of claim 1, wherein the first modifying agent is between about 0.1% by weight and about 30% by weight of crumb rubber, the second modifying agent is between about 0.5% by weight and about 30% by weight of at least one synthetic polymer, the third modifying agent is between about 0.05% by weight and about 5% by weight of at least one acid, and the fourth modifying ingredient is between about 0.0 1% and 5% of a cross-linking agent.
 5. The method of claim 1, wherein the first modifying agent is between about 0.1% by weight and about 30% by weight of crumb rubber, the second modifying agent is between about 0.05% by weight and about 5% by weight of at least one acid, the third modifying agent is between about 0.5% by weight and about 30% by weight of at least one synthetic polymer, and the fourth modifying ingredient is between about 0.01% and about 5% of a cross-linking agent.
 6. The method of claim 1, wherein the first modifying agent is between about 0.5% by weight and about 5% by weight of one or more acids, the second modifying agent is between about 0.5% by weight and about 30% by weight of at least one synthetic polymer, the third modifying agent is between about 0.1% by weight and about 30% by weight of crumb rubber, and the fourth modifying ingredient is between about 0.01% and about 5% of a cross-linking agent.
 7. A method for making a modified asphalt binder composition, comprising the steps of: a. providing neat asphalt in an appropriate vessel; b. heating the neat asphalt to a temperature of between about 120° C. and about 200° C.; c. adding to the neat asphalt between about between about 0.5% by weight and about 30% by weight crumb rubber; d. mixing the asphalt and the crumb rubber with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours e. adding between about 0.01% by weight and about 5% by weight of a cross-linking agent; f. mixing the cross-linking agent and asphalt with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours g. adding between about 0.05% by weight and about 5% by of at least one acid; and h. agitating the third modifying ingredient and asphalt with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 48 hours.
 8. A method for making a modified asphalt binder composition, comprising the steps of: a. providing neat asphalt in an appropriate vessel; b. heating the neat asphalt to a temperature of between about 120° C. and about 200° C.; c. adding to the neat asphalt between about 0.05% by weight and about 5% by weight of at least one acid; d. mixing the asphalt and the acid with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours e. adding between about 0.1% by weight and about 30% by weight of crumb rubber; f. mixing the second modifying ingredient and asphalt with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours g. adding between about 0.01% by weight and about 5% by weight of a cross-linking agent; and h. agitating the third modifying ingredient and asphalt with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 48 hours. 